transfer rates are effected by traffic fluctations at the peering points and points between as well so the only sure fire test would be to download something from a localy hosted server at the ISP and see what the transfer rate is without any other users on the LAN

512Kbs is not that high of a bandwidth for 60 users considering the maximum transfer rate without overhead is around 64k/s so basically if every user was doing something on the connection at the same moment its therortically possible each user would only have 1k/s to play with. Also consider the fact that its DSL and when you are uploading the download speed will drop as well if you have saturated your upstream connection.

I would look at the outbound / inbound traffic firstly before looking at the ISP.

Basically as traffic passes to different sites through thier ISP's depending on how saturated thier circuits are you may see an impact .
I.e if you were to connect to a website at say a small business hosting his site on a circuit which is currently being utilized and browsed by both internal and external users which has say 1.54mbs connectivity that has a load of 50% at the time you attempt to download. The max bandwidth which you could potentially utilize from that site would be 80k/s based on 1.54mb = 193k/s .

So if you are downloading from microsoft when everyone else it downloading say Sp3 for Xp ..your speeds will be far lower than say if you went to download something during mid month when its experiencing normal usage

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